1. Field of the Invention
This invention relates to the field of composite textiles, particularly to yarns, tows and structural members suitable for manufacturing fiber reinforced composites. This invention enables the production of robust pre impregnated yarns that are easily wound onto spools and processed on conventional textile machinery such as a Maypole braiding machine without difficulty.
2. Brief Description of Related Art
Several process techniques are commonly used for making fiber reinforced composites. These include:
1. Weaving, braiding, or winding of yarns (or tows) into flat fabric or shaped tubular preforms followed by saturation of the preform by liquid resin and then curing to harden the resin,
2. The fabric preforms in 1) may be stacked and painted with liquid resin, one layer at a time, stacked wet in a mold to make a solid shape, then cured under a vacuum bag or in an autoclave to harden the resin.
3. Fabric preforms in 1) may be vacuum or pressure infused with liquid resin followed by curing to harden the resin.
4. The yarns/tows may be resin impregnated (or be pre-impregnated yarns) before braiding or weaving and then cured afterwards. The resin may be partially cured before braiding and then curing is finished after braiding.
5. The yarns or tows may be resin impregnated and then used directly in a process called filament winding to make a structure, usually over a mold or mandrel, and subsequently cured to harden the resin,
6. The yarns or tows may be resin impregnated and pulled through a heated die to shape and cure the composite simultaneously (in a process called pultrusion).
7. The fibers may be chopped and sprayed simultaneously with liquid resin onto a mold. The mixture is then cured in place.
The list above is not intended to be all inclusive, and curing may or may not require heat. Liquid resin is a complex mixture of monomers, prepolymers and catalysts. Resin is usually viscous, thus limiting its ability to flow and thoroughly impregnate thick layers of compacted fiber.
The subject of this invention is included in technique number 4 above—large prepreg yarns are created from an assembly of small, thoroughly infused prepreg yarns, held together by a fibrous or polymer jacket. The large prepreg yarns are then braided or woven into a shaped composite preform and subsequently cured by heating. The prepreg yarn may be partially cured before braiding or weaving.
Fibers are infused with a polymer resin and cured to harden in order to generate a composite material. This process is relatively simple for making thin composites, but problems arise when a laminate or yarn is thick. It is difficult to infuse resin through a thick material completely to the center. To mitigate this problem, manufacturers have developed prepreg materials, which have the resin already present in the material. After making a structure, there is no required infusion, and the structure can be cured directly. Using prepreg materials will ensure that resin fills the entirety of the composite material, for maximum strength and reliability. However, prepreg yarns are particularly sticky, and therefore not generally considered suitable for braiding, weaving, or a number of other manufacturing processes.
In addition, two of the most used fibers for composite manufacture are carbon and glass. Both these fibers suffer from being excessively brittle and fragile. They break easily when subjected to abrasion in textile processing. While resin saturation of the fibers improves the abrasion resistance, it is still not good, and the resin saturation comes at the price of stickiness.
Yarns which are components in fiber reinforced composites must be thoroughly saturated with (typically a viscous) resin to make the composite. Since saturation is difficult for large assemblies of fibers, the fibrous assembly must therefore be small, or flattened into a wide, thin ribbon in order for the resin to penetrate thoroughly. These units of saturated fibers are called prepreg yarns or prepreg tows. As fiber reinforced composites are all composed of fibers infused and surrounded by a matrix resin, and given the difficulty of infusing large fiber bundles, one objective of this invention is the production of a large, thoroughly resin infused fiber bundle or yarn suitable for composite manufacture.
To make a larger yarn or structural unit, the small, resin-saturated assemblies of fibers may be combined into parallel bundles, but these bundles need to be held together. Twisting of a fiber bundle is the most common method of ensuring that fibers remain with the fiber bundle, but twisting lowers alignment of fibers in the axial direction of the yarn and reduces fiber strength. Some constraint such as a braided, wrapped or extruded overlayer can allow the fibers to remain in axial alignment while maintaining the integrity of the fiber bundle. Although the extruded overlayer is a common method of constraining a core of fibers (particularly wire) and is included in the invention, the braided or wrapped construction of the overlayer produces a more flexible yarn structure than an extruded overlayer. The winding onto spools and the interlacing on conventional equipment is more difficult with the extruded overlayer. On the other hand, the extruded overlayer is better at containing the prepreg yarn core than a braid or wrap. It is one purpose of this invention to produce very large prepreg yarns which maintain both structural integrity (a contained core with aligned filaments and no splitting), and sufficient flexibility to allow processing on conventional textile manufacturing equipment like a braiding machine
The core fibers must be contained in such a way that they are permitted to move freely around the carrier eyelets and pulleys while preserving their parallel orientation along the axis of the yarn. Another objective of this invention is to provide a jacket of minimum weight to protect, contain, and efficiently consolidate the core fibers
A particular difficulty of converting the prepreg yarns or tows into a composite structure is the stickiness of the prepreg yarn, which creates difficulty in braiding or weaving. Therefore filament winding is the preferred method of assembling the prepreg yarns or tows into a composite preform. However, the filament wound structure suffers from the lack of yarn interlacing, making the final cured composite structure subject to splitting and delamination. Another purpose of this invention is to produce a prepreg yarn that can be easily converted to an interlaced fabric or shaped preform by braiding or perhaps weaving on typical textile fabrication machinery.
Preferred fibers for reinforcing composites are often carbon and glass, because of their strength and stiffness. As both of these fibers are brittle and suffer from failure caused by abrasion, another object of this invention is the protection of these brittle fibers from abrasion damage.
Fibers are strongest in their own axial direction, but not necessarily in their other directions. Generally, when individual fibers are made into a yarn or rope, it is necessary to impart some amount of twist, in order to keep the fibers together, at least during the processes of making yarn, winding on spools and conversion into a textile structure. In the resulting yarn geometry, the axial direction of the yarn is not the same as the axial direction of many or all of the fibers. The result is a proportional reduction in strength based on the pitch of the fibers. It is another purpose of this invention to produce a braidable prepreg yarn wherein almost all of the fibers are aligned close to the yarn axis.
Both weaving and braiding provide interlaced structures. Weaving typically produces a flat fabric, while braiding can produce either flat or cylindrical fabric. Further, the cylindrical braided structure can be easily shaped to polygon structures, and is easily varied in cross sectional area and shape during braiding. Braiding is often the most desirable method for producing shaped thin composites. Therefore the products produced by this invention are particularly useful in producing braided structures.
Braided structures range in size from medical sutures and shoestrings to large marine ropes for securing ships and drilling platforms. Our examples are manufactured on typical textile braiding machines. It is anticipated that the yarn size produced by this invention will be scalable so that as the size of the braiding machine carriers and bobbins increase, the yarn size that is braidable will also increase.